This invention relates to measurement of corrosive characteristics of a fluid and more specifically concerns improved temperature compensation for such measurement.
A common method of continuous measurement of corrosion characteristics employs resistance measurements of a metallic corrodable test element to indicate, by change in resistance, the amount of metal that has been lost by corrosion over a period of time. A widely used instrument for this measurement is known as a Corrosometer probe manufactured by Rohrback Corporation, assignee of this application. One such probe employs a tubular metallic test element carrying an inner reference element made of the same material as the test element. The interior of the tubular test element is filled with a thermally conductive electrically non-conductive compound. Alternating current is passed through the elements and electrical resistance of each is measured while or after the probe has been immersed in an environment of which corrosive tendencies are to be monitored. Because resistance changes with the amount of metal in the test element, measurement of test element resistance provides an indication of corrosion. However, since resistance of the metal also changes with temperature, a reference element is provided, made of the same material as the test element and having the same temperature resistance characteristic. Thus changes in resistance of the test element that are due to long term, relatively slow temperature variation may be eliminated by comparison of resistances of the test and reference elements.
The described temperature compensation is useful solely for relatively slowly varying or long term temperature fluctuation. Dynamic or short term, rapid temperature fluctuations cause temporary but significant errors in the corrosion signal obtained with the prior long term compensation. This is due, at least in part, to the fact that the reference element, although made of the same material as the test element and placed quite close to the test element, must be protected from the corrosive environment so that the reference element does not corrode. Further, the commonly used electrically non-conductive compound, even though desirably thermally conductive, has a greater thermal resistance than does the metal of the test element. Heat of the external environment is transmitted to the protected reference element largely through this electrically insulative compound. Accordingly, a rapid rise in temperature of the environment in which the probe is immersed will effect a more rapid rate of increase of temperature of the test element than of the reference element. Upon occurrence of rapid changes in environment temperature, change in temperature of the protected reference element lags change in temperature of the unprotected test element. Therefore there is an erroneous reading, caused by this lagging temperature response of the reference element, until the temperatures of the two elements become equalized.
Although the error due to rapidly varying temperature may last for only a relatively short period, should rapid temperature changes recur frequently, the error in corrosion reading will likewise recur frequently. This error of rapid temperature fluctuation is a problem when attempting to measure corrosion in a system wherein the temperature is not held constant, a problem of increasing significance as magnitude and rate of temperature fluctuation of the fluid environment increase.
Accordingly, it is an object of the present invention to provide corrosion measurement which eliminates or substantially decreases this problem.